Question

Individuals with trisomy 21 are more likely to survive to adulthood than individuals with trisomy 18\. Based on what you know about aneuploidies from this module, what can you hypothesize about chromosomes 21 and \(18 ?\)

Short answer

Individuals with trisomy 21 (Down syndrome) have a higher survival rate than those with trisomy 18 (Edwards syndrome) because chromosome 21 may contain less critical genes and is smaller compared to chromosome 18. This results in a less detrimental impact on overall health and development. In summary, the higher survival rate of trisomy 21 individuals is likely due to a combination of less critical genes being affected and the smaller size of chromosome 21 resulting in fewer duplicated genes compared to trisomy 18.

Step-by-step solution

Understand Aneuploidies

Aneuploidies are genetic conditions characterized by an abnormal number of chromosomes. In the case of trisomy, there is an extra copy of one chromosome, leading to a total of three copies instead of the usual two. Trisomy 21 and trisomy 18 are both types of aneuploidies, with the numbers 21 and 18 referring to the affected chromosomes.

Compare Trisomy 21 and Trisomy 18

Trisomy 21, also known as Down syndrome, occurs when there is an extra copy of chromosome 21. This condition affects approximately 1 in 700 live births and is associated with intellectual disability, characteristic physical features, and an increased risk of certain medical issues. However, individuals with trisomy 21 can often live into adulthood, with many having a life expectancy beyond 60 years. On the other hand, trisomy 18, also called Edwards syndrome, is caused by an extra copy of chromosome 18. This condition is less common than trisomy 21, affecting about 1 in 5,000 live births. Trisomy 18 results in severe intellectual disability, numerous physical abnormalities, and a high mortality rate. Only around 5%-10% of affected individuals survive past their first year, and those who do live longer typically experience significant health problems throughout their lives.

Hypothesize About Chromosomes 21 and 18

Based on the information provided, we can hypothesize that the genes located on chromosome 21 may be less critical to overall survival and development than those on chromosome 18. Since individuals with trisomy 21 have a better survival rate and relatively less severe health challenges, it's possible that the extra copy of chromosome 21 has a less detrimental impact on an individual's overall health and development compared to an extra copy of chromosome 18. Additionally, chromosomes vary in size, and chromosome 21 is one of the smallest human chromosomes, whereas chromosome 18 is significantly larger. This difference in size may also partially explain the difference in survival rates. A smaller chromosome could contain fewer genes, making it less likely for the extra copy to be detrimental compared to the addition of a larger chromosome with more genes. In conclusion, the hypothesis can be that chromosome 21 may have less critical genes compared to chromosome 18, and the smaller size of chromosome 21 could result in fewer genes being duplicated, collectively leading to the higher survival rate of individuals with trisomy 21 compared to those with trisomy 18.

Key concepts

Trisomy 21

Trisomy 21 is a genetic condition often referred to as Down syndrome. It arises from the presence of an extra copy of chromosome 21, resulting in three copies rather than the typical two. This form of aneuploidy affects roughly 1 in every 700 live births and brings about a range of physical and intellectual characteristics.

• Individuals with Down syndrome typically have distinct facial features.
• They often experience developmental delays and varying degrees of intellectual disability.
• However, many people with trisomy 21 can live into adulthood, with a life expectancy stretching beyond 60 years.

Due to advances in healthcare and supportive therapies, people with Down syndrome today have the opportunity to lead fulfilling lives.
Medical concerns can include heart defects, hearing loss, and thyroid problems, but with regular medical care, many of these can be managed effectively.
Ultimately, the impact of trisomy 21 varies from individual to individual, with some people leading relatively independent lives. The support from family, community, and healthcare professionals plays a crucial role in enhancing the quality of life for those affected.

Trisomy 18

Trisomy 18, known as Edwards syndrome, is a severe chromosomal condition caused by an extra copy of chromosome 18. This condition is quite rare, affecting about 1 in every 5,000 live births. It is less common than trisomy 21 and generally results in more significant health challenges.

• Children with Edwards syndrome face severe intellectual disabilities.
• They often have multiple physical abnormalities affecting almost every organ system.
• A high mortality rate is associated with this condition, with only about 5%-10% of affected children surviving past their first year.

Those who do reach adulthood often face profound developmental delays and serious health complications.
The prognosis for trisomy 18 is typically poor, primarily due to the numerous medical challenges that arise early in life. Though the chances of survival beyond infancy are slim, dedicated medical support and care can nonetheless improve the quality of life for affected individuals.

Chromosome Abnormalities

Chromosome abnormalities refer to changes in the typical number or structure of chromosomes. These alterations can cause a variety of genetic disorders, and are classified into different types based on their nature. Aneuploidies, including trisomy 21 and trisomy 18, represent a common category of such abnormalities.

• Chromosomal abnormalities can lead to developmental and medical issues depending on which chromosomes are affected.
• They often result from errors during cell division, such as nondisjunction, that lead to an unequal distribution of chromosomes.
• Some abnormalities may cause minor physical traits, while others lead to significant health problems or affect viability.

The degree of impact often correlates with the size and genetic content of the affected chromosome. Chromosome 21, being one of the smallest human chromosomes, may result in fewer duplicated genes in trisomy 21, partly explaining the higher survival rate compared to trisomy 18. On the contrary, chromosome 18 is larger and its extra copy appears to have a more detrimental effect on health and development.
Understanding chromosome abnormalities helps in predicting outcomes and providing care tailored to each individual's needs. Genetic counseling and prenatal diagnosis can assist families in planning and managing any potential issues associated with chromosomal disorders.